628 THE PLACENTA. 



accordingly bounded on one side by a very thin, projecting, cres- 

 centic edge. These are the orifices of the uterine vessels, passing 

 into the placenta and torn off at their necks, as above described ; 

 and by carefully following them with the probe and scissors, they 

 are found to lead at once into extensive empty cavities (the pla- 

 cental sinuses), situated between the foetal tufts. We have already 

 shown that these cavities are filled during life with the maternal 

 blood ; and there is every reason to believe that before delivery, 

 and while the circulation is going on, the placenta is at least twice 

 as large as after it has been detached and expelled from the uterus. 



The placenta, accordingly, is a double organ, formed partly by 

 the chorion and partly by the decidua ; and consisting of maternal 

 and fcetal bloodvessels, inextricably entangled and united with each 

 other. 



The part which this organ takes in the development of the foetus 

 is an exceedingly important one. From the date of its formation, 

 at about the beginning of the fourth month, it constitutes the only 

 channel through which nourishment is conveyed from the mother 

 to the foetus. The nutritious materials, which circulate in abun- 

 dance in the blood of the maternal sinuses, pass through the inter 

 vening membrane by endosmosis, and enter the blood of the fcetus. 

 The healthy or injurious regimen, to which the mother is subjected, 

 will accordingly exert an almost immediate influence upon the 

 child. Even medicinal substances, taken by the mother and ab- 

 sorbed into her circulation, may readily transude through the pla- 

 cental vessels ; and they have been known in this way to exert a 

 specific effect upon the fcetal organization. 



The placenta is, furthermore, an organ of exhalation as well as 

 of absorption. The excrementitious substances, produced in the 

 circulation of the fcetus, are undoubtedly in great measure disposed 

 of by transudation through the walls of the placental vessels, to be 

 afterward discharged by the excretory organs of the mother. The 

 system of the mother may therefore be affected in this manner by 

 influences derived from the fcetus. It has been remarked more 

 than once, in the lower animals, that when the female has two sue 

 cessive litters of young by different males, the young of the second 

 litter will sometimes bear marks resembling those of the first male. 

 In these instances, the peculiar influence which produces the ex 

 ternal mark must have been transmitted by the first male directly 

 to the foetus, from the fcetus to the mother, and from the mother to 

 the foetus of the second litter. 



